Surface micromachined differential microphone

ABSTRACT

A differential microphone having a perimeter slit formed around the microphone diaphragm that replaces the backside hole previously required in conventional silicon, micromachined microphones. The differential microphone is formed using silicon fabrication techniques applied only to a single, front face of a silicon wafer. The backside holes of prior art microphones typically require that a secondary machining operation be performed on the rear surface of the silicon wafer during fabrication. This secondary operation adds complexity and cost to the micromachined microphones so fabricated. Comb fingers forming a portion of a capacitive arrangement may be fabricated as part of the differential microphone diaphragm.

RELATED APPLICATIONS

The present application is related to U.S. Pat. No. 6,788,796 forDIFFERENTIAL MICROPHONE, issued Sep. 7, 2004; and copending U.S. patentapplication Ser. No. 10/689,189 for ROBUST DIAPHRAGM FOR AN ACOUSTICDEVICE, filed Oct. 20, 2003, and Ser. No. 11/198,370 for COMB SENSEMICROPHONE, filed Aug. 5, 2005, all of which are incorporated herein byreference. This application is a division of application Ser. No.11/343,564 filed Jan. 31, 2006, now U.S. Pat. No. 7,992,283, issued Aug.9, 2011, the entirety of which is expressly incorporated herein byreference.

FUNDED RESEARCH

This work is supported in part by the following grant from the NationalInstitute of Health: R01DC005762-03. The Government may have certainrights in this invention.

FIELD OF THE INVENTION

The present invention pertains to differential microphones and, moreparticularly, to a micromachined, differential microphone absent abackside air pressure relief orifice, fabricatable using surfacemicromachining techniques.

BACKGROUND OF THE INVENTION

In typical micromachined microphones of the prior art, it is generallynecessary to maintain a significant volume of air behind the microphonediaphragm in order to prevent the back volume air from impeding themotion of the diaphragm. The air behind the diaphragm acts as a linearspring whose stiffness is inversely proportional to the nominal volumeof the air. In order to make this air volume as great as possible, andhence reduce the effective stiffness, a through-hole is normally cutfrom the backside of the silicon chip. The requirement of this backsidehole adds significant complexity and expense to such prior artmicromachined microphones. This present invention enables creation of amicrophone that does not require a backside hole. Consequently, theinventive microphone may be fabricated using only surface micromachiningtechniques.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided adifferential microphone having a perimeter slit formed around themicrophone diaphragm. Because the motion of the diaphragm in response tosound does not result in a net compression of the air in the spacebehind the diaphragm, the use of a very small backing cavity ispossible, thereby obviating the need for creating a backside hole. Thebackside holes of prior art microphones typically require that asecondary machining operation be performed on the silicon chip duringfabrication. This secondary operation adds complexity and cost to, andresults in lower yields of the microphones so fabricated. Consequently,the microphone of the present invention requires surface machining fromonly a single side of the silicon chip.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention may be obtained byreference to the accompanying drawings, when considered in conjunctionwith the subsequent, detailed description, in which:

FIG. 1 is a top view of a micromachined microphone diaphragm inaccordance with the invention;

FIG. 2 is a side, sectional, schematic view of a differential microphoneof the invention;

FIGS. 3 and 4 are, respectively, schematic representations of thedifferential microphone of FIG. 2 as a series of diaphragms without andwith an indication of the motion thereof;

FIG. 5 is a diagram showing the orientation of an incident sound wave onthe diaphragm of FIG. 1;

FIGS. 6 a-6 d are schematic representations of the stages of fabricationof the inventive, surface micromachined microphone of the invention;

FIG. 7 is a side, sectional, schematic view of a differential microphoneformed by removing a portion of a sacrificial layer of FIG. 6 d; and

FIG. 8 is a side, sectional, schematic view of an alternate embodimentof the microphone of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a micromachined differential microphoneformed by surface micromachining a single surface of a silicon chip.

The motion of a typical microphone diaphragm results in a fluctuation inthe net volume of air in the region behind the diaphragm (i.e., the backvolume). The present invention provides a microphone diaphragm designedto rock due to acoustic pressure, and hence does not significantlycompress the back volume air.

An analytical model for the acoustic response of the microphonediaphragm including the effects of a slit around the perimeter and theair in the back volume behind the diaphragm has been developed. If thediaphragm is designed to rock about a central pivot, then the backvolume and the slit has a negligible effect on the sound-inducedresponse thereof.

Referring first to FIGS. 1 and 2, there are shown, respectively, a topview of a micromachined microphone diaphragm, including a slit aroundthe perimeter of the diaphragm, and a side, sectional, schematic view ofa differential microphone in accordance with the invention, generally atreference number 100. A rigid diaphragm 102 is supported by hinges 104that form a pivot point 106 around which diaphragm 102 may “rock” (i.e.,reciprocally rotate). A back volume of air 108 is formed in a cavity 110formed in the chip substrate 112. A slit 114 is formed between theperimeter 103 of diaphragm 102 and the chip substrate 112.

Diaphragm 102 rotates about the pivot point 106 due to a net moment thatresults from the difference in the acoustic pressure that is incident onthe top surface portions 116, 118 that are separated by the centralpivot point 106.

In order to more readily examine the effects of the back volume 108 andthe slit 114 around the diaphragm 102, several assumptions are made. Itis assumed that the pivot point 106 is centrally located and thatdiaphragm 102 is designed such that the rocking, or out-of-phase motionof diaphragm 102 is the result of the pressure difference on the twoportions 116, 118 of the exterior surface thereof. Because diaphragm 102is normally designed to respond to the difference in pressure on its twoportions 116, 118, microphone 100 is referred to it as a differentialmicrophone. However, in addition to motion induced by pressuredifferences, it is also possible that diaphragm 102 will be deflecteddue to the average pressure on its exterior surface. Such pressurecauses diaphragm 102 motion in which both portions 116, 118 of thediaphraym 102 separated by the pivot point 106 respond in-phase.

The air 108 a in the slit 114 around the diaphragm 102 on each portion116, 118 is assumed to have a mass ma. Consequently, diaphragm 102responds like an oscillator. Hence, the two portions 116, 118 of thedifferential microphone 100, along with the two masses of air 108, 108 acan be represented by a system of diaphragms 120, 122, 124, 126 as shownin FIG. 3. Each of the diaphragms is identified as air 108 (referencenumber 120), microphone portion 116 (reference number 122), microphoneportion 118 (reference number 124), and air 108 a (reference number126). The response of each diaphragm is governed by the followingequation:

m _(i) {umlaut over (X)} _(i) +k _(i) X _(i) =F _(i)  (1)

where: F_(i) is the net force acting on each diaphragm 120, 122, 124,126 and X₄, X₁, X₂, and X₃, represent the motion of each respectivediaphragm 120, 122, 124, 126. As may be seen in FIG. 4, X₁ and X₂represent the average motion of each portion 116, 118 of the diaphragmand X₃ and X₄ represent the motion of the air 108 a in the slit 114.

A differential microphone without the slit 114 (i.e., a differentialmicrophone of the prior art) can be represented by a two degree offreedom system with rotational response θ and translational response x:

m{umlaut over (x)}+kx=F  (2a)

I{umlaut over (θ)}+k _(t) θ=M  (2b)

where: F is the net applied force, and M is the resulting moment aboutthe pivot point. k and k_(t) represent the effective transversemechanical stiffness and the torsional stiffness respectively, of thediaphragm and pivot 102, and 106.

If d is the distance between the centers of each portion 116, 118 of thediaphragm 102, then X₁ and X₂ may be expressed in terms of thegeneralized co-ordinates x and θ:

$\begin{matrix}{{X_{1} = {x + {\frac{d}{2}\theta}}}{and}{X_{2} = {\left. {x - {\frac{d}{2}\theta}}\Rightarrow x \right. = \frac{X_{1} + X_{2}}{2}}}{and}{\theta = \frac{X_{1} - X_{2}}{2}}} & (3)\end{matrix}$

These relations may also be written in matrix form:

$\begin{matrix}{\begin{pmatrix}X_{1} \\X_{2} \\X_{3} \\X_{4}\end{pmatrix} = {{\begin{bmatrix}{d/2} & 1 & 0 & 0 \\{{- d}/2} & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}\begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}} = {\lbrack T\rbrack \begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}}} & (4)\end{matrix}$

If the dimensions of the air cavity 110 (FIG. 2) behind the diaphragm102 are much smaller than the wavelength of sound, it may be assumedthat the air pressure in the back volume 108 is spatially uniform withinthe air cavity. The air 108 in this back volume (i.e., cavity 110) thenacts as a linear spring. It is necessary to relate the pressure in theback volume air 108 to the displacement of the diaphragm 102 to estimatethe stiffness of this spring. If the mass of the air in back volume 108is assumed to be constant, then the motion of the diaphragm 102 resultsin a change in the density of the air 108 in cavity 110. The relationbetween the acoustic, or fluctuating density, ρ_(a) and the acousticpressure, p, is the equation of state:

p=c ²ρ_(a)  (5)

where: c is the speed of sound.

The total density of air is the mass divided by the volume, ρ=M/V. Ifthe volume fluctuates by an amount ΔV due to the motion of diaphragm102, then the density becomes ρ=M/(V+ΔV)=M/V(1+ΔV/V. For small changesin the volume, this can be expanded in a Taylor's series

=p≈(M/V)(1−ΔV/V). The acoustic fluctuating density is thenρ_(a)=−ρ₀ΔV/V, where the nominal density is ρ₀=M/V. The fluctuatingpressure in the volume V due to the fluctuation ΔV, resulting from anoutward motion, x, of the diaphragm 102 is then given by:

P _(d)=ρ₀ c ² ΔV/V=−ρ ₀ c ² Ax/V  (6)

where: A is half the area of the diaphragm.

This pressure in the back volume 108 exerts a force on the diaphragm 102given by:

F _(d) =P _(d) A=ρ ₀ c ² A ² x/V=−K _(d) x  (7)

where: K_(d)=ρ₀c²A²/V is the equivalent spring constant of the air 108with units of N/m.

The force due to the back volume of air 108 adds to the restoring forcefrom the mechanical stiffness of the diaphragm 102. Including the air inthe back volume 108, Equation (2) becomes:

m{umlaut over (x)}+kx+k _(d) x=−PA  (8)

The negative sign on the right hand side of Equation (8) is attributedto the convention that a positive pressure on the diaphragm's exteriorcauses a force in the negative direction. From Equation (8), themechanical sensitivity at frequencies well below the resonant frequencyis given by S_(m)=A/(k+K_(d)) m/Pa.

The air 108 a in the slit or vent 114 is forced to move due to thefluctuating pressures both within the space 110 behind the diaphragm 102and in the external sound field, not shown. Again, it may be assumedthat the dimensions of the volume of moving air in the slit 114 to bemuch smaller than the wavelength of sound and hence it may beapproximately represented as a lumped mass ma. An outward displacement,x_(a), of the air 108 a in the slit 114 causes a change in the volume ofair in the back volume 108. A corresponding pressure similar to Equation(6) is given by:

P _(aa)=−ρ₀ c ² A _(a) x _(a) /V  (9)

where: A_(a) is the area of the slit 114 on which the pressure acts.

Again, the pressure due to motion of air 108 a in the slit 114 applies arestoring force on the mass thereof given by:

F _(aa) =P _(aa) A _(a)=−ρ₀ c ² A ² x _(a) /V=−K _(aa) x _(a)  (10)

Since the pressure in the back volume 108 is nearly independent ofposition within the back volume, a change in the pressure due to motionof the air 108 a in the slit 114 exerts a force on the diaphragm 102given by:

F _(ad) =P _(aa) A=−ρ ₀ c ² A _(a) Ax _(a) /V=−K _(ad) x _(a)  (11)

Similarly, the motion of the diaphragm causes a force on the mass of air108 given by:

F _(da) =P _(d) A _(a)=−ρ₀ c ² AA _(a) x/V=−K _(da) x  (12)

From Equations (6), (10), (11) and (12), it may be seen that the forcesadd to the restoring forces due to mechanical stiffness in the system ofEquation (1). Hence the volume change due to motion of each co-ordinateis given by ΔV_(i)=A_(i)X_(i) and F_(i)=PA_(i). Now, the total pressuredue to the motion of all co-ordinates is given by:

$\begin{matrix}{P_{tot} = {{{- \frac{\rho_{0}c^{2}}{V}}\left( {{A_{1}X_{1}} + {A_{2}X_{2}} + {A_{3}X_{3}} + {A_{4}X_{4}}} \right)} = {{- \frac{\rho_{0}c^{2}}{V}}{\sum\limits_{i}\; {A_{i}X_{i}}}}}} & (13)\end{matrix}$

The force due to this pressure on the jth coordinate in this model(indicating the motions of 120, 122, 124, and 126 in FIG. 3) is thengiven by:

$\begin{matrix}{F_{j} = {{P_{tot}A_{j}} = {{\left( {{- \frac{\rho_{0}c^{2}}{V}}{\sum\limits_{i}\; {A_{i}X_{i}}}} \right)A_{j}} = {- {\sum\limits_{i}\; {K_{ij}X_{i}}}}}}} & (14)\end{matrix}$

where:

$K_{ij} = {{- \frac{\rho_{0}c^{2}}{V}}A_{i}{A_{j}.}}$

Equation (14) may be written as:

$\begin{matrix}{\begin{pmatrix}F_{1} \\F_{2} \\F_{3} \\F_{4}\end{pmatrix} = {{- \begin{bmatrix}K_{11} & K_{12} & K_{13} & K_{14} \\K_{21} & K_{22} & K_{23} & K_{24} \\K_{31} & K_{32} & K_{33} & K_{34} \\K_{41} & K_{42} & K_{43} & K_{44}\end{bmatrix}}\begin{pmatrix}X_{1} \\X_{2} \\X_{3} \\X_{4}\end{pmatrix}}} & (15)\end{matrix}$

Combining Equations (4) and (15), in terms of the coordinates θ and x ofthe differential microphone, the force is represented as:

$\begin{matrix}{\begin{pmatrix}F_{1} \\F_{2} \\F_{3} \\F_{4}\end{pmatrix} = {{- {\begin{bmatrix}K_{11} & K_{12} & K_{13} & K_{14} \\K_{21} & K_{22} & K_{23} & K_{24} \\K_{31} & K_{32} & K_{33} & K_{34} \\K_{41} & K_{42} & K_{43} & K_{44}\end{bmatrix}\lbrack T\rbrack}}\begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}} & (16)\end{matrix}$

Equation (16) may be rewritten in terms of the average force acting onthe differential microphone 100 and the net moment acting on the pivotpoint 106. This is given by:

$F = \frac{F_{1} + F_{2}}{2}$ and$M = {\left. {\left( {F_{1} - F_{2}} \right)\frac{d}{2}}\Rightarrow F_{1} \right. = {F + \frac{M}{d}}}$and $F_{2} = {F - \frac{M}{d}}$

What follows therefrom is:

$\begin{matrix}{\mspace{79mu} {\begin{pmatrix}M \\F \\F_{3} \\F_{4}\end{pmatrix} = {\left. {\begin{bmatrix}{d/2} & {{- d}/2} & 0 & 0 \\{1/2} & {1/2} & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}\begin{pmatrix}F_{1} \\F_{2} \\F_{3} \\F_{4}\end{pmatrix}}\Rightarrow\begin{pmatrix}M \\F \\F_{3} \\F_{4}\end{pmatrix} \right. = {\left. {{- {{\begin{bmatrix}{d/2} & {{- d}/2} & 0 & 0 \\{1/2} & {1/2} & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}\begin{bmatrix}K_{11} & K_{12} & K_{13} & K_{14} \\K_{21} & K_{22} & K_{23} & K_{24} \\K_{31} & K_{32} & K_{33} & K_{34} \\K_{41} & K_{42} & K_{43} & K_{44}\end{bmatrix}}\lbrack T\rbrack}}\begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}\mspace{79mu}\Rightarrow\begin{pmatrix}M \\F \\F_{3} \\F_{4}\end{pmatrix} \right. = {{- \left\lbrack K^{\prime} \right\rbrack}\begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}}}}} & (17)\end{matrix}$

Hence, the system of equations:

$\begin{matrix}{{{\begin{bmatrix}I & 0 & 0 & 0 \\0 & m & 0 & 0 \\0 & 0 & m_{a} & 0 \\0 & 0 & 0 & m_{a}\end{bmatrix}\begin{pmatrix}\overset{¨}{\theta} \\\overset{¨}{x} \\{\overset{¨}{X}}_{3} \\{\overset{¨}{X}}_{4}\end{pmatrix}} + {\begin{bmatrix}k_{t} & 0 & 0 & 0 \\0 & k & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix}\begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}} = {\left. {\begin{pmatrix}M \\F \\F_{3} \\F_{4}\end{pmatrix} - {\left\lbrack K^{\prime} \right\rbrack \begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}}\Rightarrow{{\begin{bmatrix}I & 0 & 0 & 0 \\0 & m & 0 & 0 \\0 & 0 & m_{a} & 0 \\0 & 0 & 0 & m_{a}\end{bmatrix}\begin{pmatrix}\overset{¨}{\theta} \\\overset{¨}{x} \\{\overset{¨}{X}}_{3} \\{\overset{¨}{X}}_{4}\end{pmatrix}} + {\left\{ {\begin{bmatrix}k_{t} & 0 & 0 & 0 \\0 & k & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix} + \left\lbrack K^{\prime} \right\rbrack} \right\} \begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}} \right. = \begin{pmatrix}M \\F \\F_{3} \\F_{4}\end{pmatrix}}} & (18)\end{matrix}$

It is important to note that the coupling between the coordinates inEquation (18) is due to the matrix [K′]. Evaluating the elements of [K′]from equations (4) and (17), the governing equation for the rotation, θ,of the diaphragm is:

$\begin{matrix}{{{I\overset{¨}{\theta}} + {\left( {k_{t} + {\left( \frac{d}{2} \right)^{2}\left( {k_{11} - k_{12} - k_{21} + k_{22}} \right)}} \right)\theta} + {\left( \frac{d}{2} \right)\left( {k_{11} + k_{12} - k_{21} - k_{22}} \right)x} + {\left( \frac{d}{2} \right)\left( {k_{13} - k_{23}} \right)X_{3}} + {\left( \frac{d}{2} \right)\left( {k_{14} - k_{24}} \right)X_{4}}} = M} & (19)\end{matrix}$

where:

$K_{ij} = {{- \frac{\rho_{0}c^{2}}{V}}A_{i}{A_{j}.}}$

Note that if the diaphragm is symmetric, A₁=A₂, and A₃=A₄. As a result,the coefficients of x, X₃, and X₄ in equation (19) become zero. Thiscauses the governing equation for rotation to be independent of theother coordinates as well as independent of the volume, V (i.e.,I{umlaut over (θ)}+k_(t)θ=M). The rotation is also independent of thearea of the slits 114, because of the assumption that the pressurecreated within the back volume 108 is spatially uniform and thereforedoes not create any net moment on the diaphragm 102.

In the foregoing analysis, it has been assumed that the microphonediaphragm 102 is symmetric about the central pivot point 106. Asmentioned above, in this case, the diaphragm 102 behaves like adifferential microphone diaphragm and has a first-order directionalresponse. If, however, the diaphragm 102 is designed to be asymmetricalwith respect to pivot point 106, then the directionality departs fromthat of a differential microphone and tends toward that of anondirectional microphone. The effect of the back volume 108 on therotation of the diaphragm 102 can be determined by extending theforegoing analysis to this non-symmetric case.

In the following, expressions are derived for the forces and moment thatare applied to the microphone diaphragm 102 due to an acoustic planewave. For plane waves, the pressure acting on the diaphragm 102 isassumed to be of the form p=Pe^(ît)e^((−îk) ^(x) ^(x−îk) ^(y) ^(y)),where

${k_{x} = {\frac{\omega}{c}\sin \; {\varphi sin}\; \theta}},{k_{y} = {\frac{\omega}{c}\sin \; {\varphi cos}\; \theta}}$and ${k_{z} = {\frac{\omega}{c}\cos \; \varphi}},$

where the angles are defined in FIG. 5. The net moment due to theincident sound is given by

$M = {\int_{{- L_{x}}/2}^{L_{x}/2}{\int_{{- L_{y}}/2}^{L_{y}/2}{P\; ^{\hat{}\; \omega \; t}^{({{{- \hat{}}k_{x}x} - {\hat{i}k_{y}y}})}x{x}{y}}}}$

where L_(x) and L_(y) are the lengths in the x and y directions,respectively.

The expression for the moment can be integrated separately over the xand y directions to give

$\left. \Rightarrow M \right. = {P\; ^{\hat{}\omega \; t}{\int_{{- L_{x}}/2}^{L_{x}/2}{^{{- \hat{}}k_{x}x}x{x}{\int_{{- L_{y}}/2}^{L_{y}/2}{^{{- \hat{}}k_{y}y}{{y}.}}}}}}$

Integrating over the y coordinatebecomes

$\left. \Rightarrow M \right. = {\left. {P\; ^{\hat{}\omega \; t}\frac{\left( {^{\hat{}k_{y}{L_{y}/2}} - ^{\hat{}k_{y}{L_{y}/2}}} \right.}{{- }\; k_{y}}{\int_{{- L_{x}}/2}^{L_{x}/2}{^{{- \hat{}}k_{x}x}x{x}}}}\Rightarrow M \right. = {P\; ^{\hat{}\omega \; t}\frac{2\; {\sin\left( \frac{k_{y}L_{y}}{2} \right.}}{\;}{\int_{{- L_{x}}/2}^{L_{x}/2}{^{{- \hat{}}k_{x}x}x{{x}.}}}}}$

Integrating by parts for the x-component gives:

$\left. \Rightarrow M \right. = {P\; ^{\hat{}\; \omega \; t}{{\frac{2\; {\sin \left( \frac{k_{y}L_{y}}{2} \right)}}{k_{y}}\left\lbrack {{\frac{L_{x}}{2}\frac{\left( {^{{- \hat{}}k_{x}{L_{x}/2}} + ^{\hat{}k_{x}{L_{x}/2}}} \right)}{{- }\; k_{x}}} + {\frac{1}{k_{x}^{2}}\left( {^{\hat{}k_{x}{L_{x}/2}} + ^{{- \hat{}}k_{x}{L_{x}/2}}} \right)}} \right\rbrack}.}}$

Simplifying the above gives:

$\begin{matrix}{\left. \Rightarrow M \right. = {P\; {{^{\hat{}\; \omega \; t}\left\lbrack \frac{2\; {\sin \left( \frac{k_{y}L_{y}}{2} \right)}}{k_{y}} \right\rbrack}\left\lbrack {{\frac{- L_{x}}{\hat{}k_{x}}{\cos \left( \frac{k_{x}L_{x}}{2} \right)}} - {\frac{2\; \hat{i}}{k_{x}^{2}}{\sin \left( \frac{k_{x}L_{x}}{2} \right)}}} \right\rbrack}}} & (20)\end{matrix}$

Because the dimensions of the diaphragm are very small relative to thewavelength of sound, the arguments of the sin and cosine functions arevery small, which results in

${\sin \left( \frac{k_{y}L_{y}}{2} \right)} \approx {\frac{k_{y}L_{y}}{2}.}$

The second term in brackets in Equation (20) is expanded to second orderusing Taylor's series. Using

${{\cos \; \theta} \approx {1 - {\frac{\theta^{2}}{2}\mspace{14mu} {and}\mspace{14mu} \sin \; \theta}} \approx {\theta - \frac{\theta^{3}}{6}}},$

in Equation (16),

$M \approx {P\; {{{^{\hat{}\omega \; t}\left\lbrack {2\left( \frac{L_{y}}{2} \right)} \right\rbrack}\left\lbrack {{\frac{- L_{x}}{\hat{}k_{x}}\left( {1 - \frac{k_{x}^{2}L_{x}^{2}}{8}} \right)} - {\frac{2\; \hat{}}{k_{x}^{2}}\left( {\frac{k_{x}L_{x}}{2} - \frac{k_{x}^{3}L_{x}^{3}}{48}} \right)}} \right\rbrack}.}}$

Simplifying gives:

$\begin{matrix}{M \approx {P\; ^{\hat{}\omega \; t}L_{y}\frac{k_{x}L_{x}^{3}}{12\; \hat{}}}} & (21)\end{matrix}$

The net force is given by a surface integral of the acoustic pressure,

$F = {- {\int_{{- L_{x}}/2}^{L_{x}/2}{\int_{{- L_{y}}/2}^{L_{y}/2}{P\; ^{\hat{}\omega \; t}^{{- \hat{}}k_{x}{x--}\hat{}k_{y}y}{x}{{y}.}}}}}$

Carrying out the integration gives:

${F--}P\; ^{\hat{}\omega \; t}\frac{2\; {\sin \left( \frac{k_{x}L_{x}}{2} \right)}}{k_{x}}{\frac{2\; \sin \left( \frac{k_{y}L_{y}}{2} \right)}{k_{y}}.}$

Again, for small angles this becomes

F=−Pe ^(îωt)(L _(x) L _(y))  (22)

Using Equations (15), (18) and (19):

${{\begin{bmatrix}I & 0 & 0 & 0 \\0 & m & 0 & 0 \\0 & 0 & m_{a} & 0 \\0 & 0 & 0 & m_{a}\end{bmatrix}\begin{pmatrix}\overset{..}{\theta} \\\overset{..}{x} \\{\overset{..}{X}}_{3} \\{\overset{..}{X}}_{4}\end{pmatrix}} + {\left\{ {\begin{bmatrix}k_{t} & 0 & 0 & 0 \\0 & k & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix} + \left\lbrack K^{\prime} \right\rbrack} \right\} \begin{pmatrix}\theta \\x \\X_{3} \\X_{4}\end{pmatrix}}} = \begin{pmatrix}{P\; ^{\hat{}\omega \; t}L_{y}\frac{k_{x}L_{x}^{3}}{12\; \hat{}}} \\{{- P}\; {^{\hat{}\omega \; t}\left( {L_{x}L_{y}} \right)}} \\{- {PA}_{a}} \\{- {PA}_{a}}\end{pmatrix}$

Let

$K_{eq} = \left\{ {\begin{bmatrix}k_{t} & 0 & 0 & 0 \\0 & k & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix} + \left\lbrack K^{\prime} \right\rbrack} \right\}$

and assume θ=Θe^(îωt), x=Xe^(îωt), X₃=X₃e^(îωt) and

$\begin{matrix}{X_{4} = \left. {X_{4}^{\hat{}\omega \; t}}\Rightarrow{\quad{{\begin{bmatrix}{{K_{eq}\left( {1,1} \right)} - {I\; \omega^{2}}} & {K_{eq}\left( {1,2} \right)} & {K_{eq}\left( {1,3} \right)} & {K_{eq}\left( {1,4} \right)} \\{K_{eq}\left( {2,1} \right)} & {{K_{eq}\left( {2,2} \right)} - {m\; \omega^{2}}} & {K_{eq}\left( {2,3} \right)} & {K_{eq}\left( {2,4} \right)} \\{K_{eq}\left( {3,1} \right)} & {K_{eq}\left( {3,2} \right)} & {{K_{eq}\left( {3,3} \right)} - {m_{\alpha}\; \omega^{2}}} & {K_{eq}\left( {3,4} \right)} \\{K_{eq}\left( {4,1} \right)} & {K_{eq}\left( {4,2} \right)} & {K_{eq}\left( {4,3} \right)} & {{K_{eq}\left( {4,4} \right)} - {m_{\alpha}\; \omega^{2}}}\end{bmatrix}\begin{pmatrix}{\Theta/P} \\{X/P} \\{X_{3}/P} \\{X_{4}/P}\end{pmatrix}} = \begin{pmatrix}{L_{y}\frac{k_{x}L_{x}^{3}}{12\; \hat{}}} \\{- \left( {L_{x}L_{y}} \right)} \\{- A_{a}} \\{- A_{a}}\end{pmatrix}}} \right.} & (23)\end{matrix}$

Using Equation (23), the displacement and rotation relative to theamplitude of the pressure, X/P and θ/P, as a function of the excitationfrequency, ω may be computed.

Based on the foregoing analysis, it may be observed that if the air inthe back volume 108 is considered to be in viscid, the performance ofthe differential microphone diaphragm 102 is not degraded if the depthof the backing cavity 110 is reduced significantly. Thus the microphone100 can be fabricated without the need for a backside hole behind thediaphraym 102. The fabrication process for the surface micromachinedmicrophone diaphragm is shown in FIGS. 6 a-6 d.

Referring now to FIG. 6 a, there is shown a bare silicon wafer 200before fabrication is begun. Such silicon wafers are known to thoseskilled in the art and are not further described herein.

As may be seen in FIG. 6 b, a sacrificial layer (e.g., silicon dioxide)202 is deposited on an upper surface of wafer 200. While silicon dioxidehas been found suitable for forming sacrificial layer 202, many othersuitable material are know to those of skill in the art. For example,low temperature oxide (LTO), phosphosilicate glass (PSG), aluminum areknown to be suitable. Likewise, photoresist material may be used. Instill other embodiments, polymeric materials may be used to formsacrificial layer 202. It will be recognized that other suitablematerial may exist. The choice and use of such material is considered tobe known to those of skill in the art and is not further describedherein. Consequently, the invention is not considered limited to aspecific sacrificial layer material. Rather, the invention covers anysuitable material used to form a sacrificial layer in accordance withthe inventive method.

Over sacrificial layer 202, a layer of structural material (for examplepolysilicon) is also deposited. While polysilicon has been foundsuitable for the formation of layer 204, it will be recognized thatlayer 204 may be formed from other materials. For example, siliconnitride, gold, aluminum, copper or other material having similarcharacteristic may be used. Consequently, the invention is not limitedto the specific material chosen for purposes of disclosure but coversany and all similar, suitable material. Layer 204 will ultimately formdiaphragm 102 (FIG. 2).

As is shown in FIG. 6 c, the diaphragm material, layer 204 is nextpatterned and etched to form the diaphragm 102, leaving slits 114.

Finally, as may be seen in FIG. 6 d, the sacrificial layer 202 underdiaphragm 102 is removed leaving cavity 110. After the removal of thesacrificial layer, the microphone diaphragm 102 has a back volume 108with a depth equal to the thickness of the sacrificial layer 202. Themicrophone is shown schematically in FIG. 7.

To convert motion of diaphragm 102 into an electronic signal, combfingers incorporated at 208 (FIG. 7) may be integrated with thediaphragm. Such comb or interdigitated fingers are described in detailin copending U.S. patent application Ser. No. 11/198,370 for COMB SENSEMICROPHONE, filed Aug. 5, 2005.

As an alternative sensing scheme, the fundamental microphone structureof FIG. 7 may be modified slightly to include two conductive layers 206disposed between silicon chip 200 and additional conductive layer 204 toform back plates forming fixed electrodes of capacitors. These backplates are electrically separated from each other in order to allowdifferential capacitive sensing of the diaphragm motion.

It should be noted that one could employ both the comb fingers 208 andthe back plate 206 to perform capacitive sensing. In this case, inaddition to serving as an element of a capacitive sensing arrangement, avoltage applied to comb sense fingers 208 may be used to stabilizediaphragm 102. The voltage applied between the comb fingers and thediaphragm can be used to reduce the effect of the collapse voltage,which is a common design issue in conventional back plate-basedcapacitive sensing schemes.

It will be recognized that many other sensing arrangements may be usedto convert motion of diaphragm 102 to an electrical signal.Consequently, the invention is not limited to any particular diaphragmmotion sensing arrangement.

Since other modifications and changes varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, the invention is not considered limited to the example chosen forpurposes of disclosure, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention.

Having thus described the invention, what is desired to be protected byLetters Patent is presented in the subsequently appended claims.

1. A method of forming a miniature, surface micromachined, differentialmicrophone, comprising: depositing a sacrificial layer on a top surfaceof a silicon wafer; depositing a diaphragm material layer on an uppersurface of said sacrificial layer; etching said diaphragm material layerto isolate a diaphragm therein; and removing at least a portion of saidsacrificial layer from a region beneath said isolated diaphragm.
 2. Themethod according to claim 1, wherein said etching further comprisesforming comb sense fingers along at least a portion of a perimeter ofsaid diaphragm.
 3. The method according to claim 1, further comprisingforming a conductive layer intermediate said top surface of said siliconwafer and said sacrificial layer.
 4. The method according to claim 1,wherein said depositing a sacrificial layer comprises depositing a layerof at least one material selected from the group consisting of silicondioxide, low temperature oxide (LTO), phosphosilicate glass (PSG),aluminum, photoresist material, and a polymeric material.
 5. The methodaccording to claim 1, wherein said depositing a diaphragm material layercomprises depositing a layer of at least one material selected from thegroup consisting of polysilicon, silicon nitride, gold, aluminum, andcopper.
 6. The method according to claim 1, further comprisingdepositing a conductive material on the diaphragm material layer,configured to serve as an electrode for sensing an acoustic vibration ofthe diaphragm.
 7. The method according to claim 1, wherein the diaphragmis isolated from a remaining portion of said diaphragm material layerafter etching and removing, by a slit formed adjacent to a perimeter ofsaid diaphragm, and another portion comprising a supporting hingeattaching the diaphragm to the remaining portion of the diaphragmmaterial layer.
 8. The method according to claim 7, wherein an enclosedback volume is formed beneath said diaphragm having a depth defined by athickness of said sacrificial layer, said back volume communicating witha region external thereto only via said slit.
 9. The method according toclaim 8, further comprising forming a plurality of comb sense fingersdisposed along at least a portion of a perimeter of said diaphragm. 10.The method according to claim 1, further comprising depositing aconductive layer intermediate said top surface of said silicon wafer anda lower surface of said sacrificial layer.
 11. The method according toclaim 1, wherein said etching comprises forming a narrow slit around aportion of the diaphragm to separate the diaphragm from a remainingportion of the diaphragm material layer, the slit being configured todefine corresponding sets of comb sense fingers on the diaphragm and theremaining portion of the diaphragm material layer, and maintaining aportion bridging the diaphragm and the remaining portion of thediaphragm material layer which is configured to act as a resilienthinge, wherein the diaphragm moves in response to acoustic vibrationabout the resilient hinge, further comprising providing respectivelyisolated electrodes on the corresponding sets of comb sense fingers forcapacitive sensing of the diaphragm movement.
 12. The method accordingto claim 1, wherein said removing defines an enclosed back volume forthe diaphragm.
 13. A method, comprising: depositing said sacrificiallayer on an upper surface of a silicon wafer; depositing a diaphragmmaterial on the upper surface of said sacrificial layer; etching saiddiaphragm material layer to isolate said diaphragm therein by forming aslit defining a plurality of comb sense fingers along at least a portionof a perimeter of said diaphragm and a supporting hinge; removing atleast a portion of said sacrificial layer from a region beneath saiddiaphragm to form said back volume; forming a hole through the siliconwafer or a remaining portion of the sacrificial layer allowing fluidiccommunication between the back volume and a region external thereto. 14.The method according to claim 13, further comprising forming aconductive layer intermediate said upper surface of said silicon waferand said sacrificial layer.
 15. The method according to claim 13,wherein said depositing a sacrificial layer comprises depositing a layerof at least one material selected from the group consisting of silicondioxide, low temperature oxide (LTO), phosphosilicate glass (PSG),aluminum, photoresist material, and a polymeric material.
 16. The methodaccording to claim 13, wherein said depositing a diaphragm materiallayer comprises depositing a layer of at least one material selectedfrom the group consisting of polysilicon, silicon nitride, gold,aluminum, and copper.
 17. A method, comprising: depositing on a surfaceof a substrate a sacrificial layer, and a diaphragm layer disposed ontop of said sacrificial layer; forming an aperture through saiddiaphragm layer resulting in at least one support; removing at least aportion of said sacrificial layer beneath the diaphragm layer, resultingin a pivotally supported diaphragm with a void between said diaphragmlayer and said substrate maintained over the void by the at least onesupport, wherein said diaphragm has an axis of rotational movement inresponse to a torque about the at least one support; and forming aplurality of comb sense fingers disposed along at least a portion of aperimeter of said diaphragm as a transducer for producing an electricalsignal responsive to a displacement of said diaphragm with respect tosaid substrate due to an acoustic force exerting the torque on thediaphragm.
 18. The method according to claim 17, wherein said axis ofrotational movement is located such that said diaphragm has adirectional response to an acoustic wave.
 19. The method according toclaim 18, wherein a volume beneath said diaphragm is substantiallyconstant with respect to the rotational movement in response to theacoustic force.
 20. The method according to claim 17, wherein the voidbeneath said diaphragm has a depth approximately the same as a thicknessof said sacrificial layer.
 21. The microphone according to claim 17,wherein said axis is located such that said diaphragm has a directionalresponse to an acoustic force, and wherein a volume of the void beneathsaid diaphragm is substantially constant with respect to movements inresponse to the acoustic force, said aperture comprising a slitpermitting air flow therethrough, and a moment M acting on one side ofsaid diaphragm with respect to said axis, in response to the acousticforce produced by an acoustic wave having a wavelength larger than amaximum linear dimension of said void, over a small angle of deflection,is approximately:$M = {P\; ^{\; \omega \; t}L_{y}\frac{k_{x}L_{x}^{3}}{12\; \hat{i}}}$in which: L_(y) is a dimension of the diaphragm along said axis, L_(x)is a dimension of the diaphragm perpendicular to, and measured from saidaxis in a plane of the diaphragm, P represents an amplitude of theacoustic wave, ω represents a frequency of the acoustic wave, crepresents a velocity of the acoustic wave, k_(x)=(ω/c)sin φ sin θ, φ isthe angle between a plane of the diaphragm and the propagation of theacoustic wave, and θ is the angle of propagation of the acoustic waveprojected onto the plane of the diaphragm.